Traditional (non-lithium containing) aluminum alloys have been semi-continuously cast in open bottom molds since the invention of Direct Chill casting in the 1938 by the Aluminum Company of America (now Alcoa). Many modifications and alterations to the process have occurred over the years since then, but the basic process remains essentially the same. Those skilled in the art of aluminum ingot casting will understand that new innovations improve the process, while maintaining its general functions. From the beginning of the use of this process, water has been used as the coolant of preference to chill the open-bottomed mold which provides the primary cooling in forming the solid ingot shell and also to be used to provide the secondary cooling of the ingot shell below the bottom of the mold.
Unfortunately, there is an inherent risk from a “bleed-out” or “run-out” during the casting process. Due to the inherent nature of the process the perimeter of the ingot comprises a thin shell of solidified metal holding an inner cavity of partially solidified and liquid molten metal that will bleed-thru the ingot shell if there is an occurrence where the aluminum ingot being cast is not properly solidified. Molten aluminum can then come into contact with the water coolant in various locations in the casting pit (e.g. between the ingot butt or bottom and the starting block, on the metal (usually steel) bottom block base, the pit walls or at the bottom of the pit) as well as in the ingot cavity where the water can enter through a rupture in the ingot shell below the bottom of the mold. Water during a “bleed-out” or “run-out” can cause an explosion from (1) conversion of water to steam from the thermal mass of the aluminum heating the water to >212° F. or (2) the chemical reaction of the molten metal with the water resulting in release of energy causing a chemical reaction generated explosion.
U.S. Pat. No. 4,651,804 describes a more modern aluminum casting pit design. According to this reference, it has become standard practice to mount the metal melting furnace slightly above ground level with the casting mould at, or near to, ground level and lower the cast ingot into a water containing pit as the casting operation proceeds. Cooling water from the direct chill flows into the pit and is continuously removed there-from while leaving a permanent deep pool of water within the pit. This process remains in current use and, throughout the world, probably in excess of 5 million tons of aluminum and its alloys are produced annually by this method. However, the use of this permanent deep pool of water does not prevent all explosions from occurring in a casting pit, since explosions can still occur in other locations in the casting pit such as mentioned above where there is still water coming into contact with molten aluminum. In spite of these improvements, there are still a significant number of explosions during the casting process each year even with use of deep pool water pits.
With the advent of aluminum lithium alloys the danger of explosions has increased further, because some of the preventive measures typically used for minimizing the potential for molten aluminum and water explosions are no longer sufficient. Again referencing U.S. Pat. No. 4,651,804, in the last several years, there has been growing interest in light metal alloys containing lithium. Lithium makes the molten alloys more reactive. In a “Metal Progress” article, May 1957, pages 107 to 112, (hereinafter referred to as “Long”), Long refers to previous work by H. M. Higgins who had reported on aluminum/water reactions for a number of alloys including Al—Li and concluded that “When the molten metals were dispersed in water in any way . . . Al—Li alloy . . . underwent a violent reaction.” It has also been announced by the Aluminum Association Inc. (of America) that there are particular hazards when casting such alloys by the direct chill process. The Aluminum Company of America has subsequently published video recordings of tests that demonstrate that such alloys can explode with great violence when mixed with water.
Other work has demonstrated that the explosive forces associated with adding lithium to aluminum alloys can increase the nature of the explosive energy several times that for aluminum alloys without lithium. When molten aluminum alloys containing lithium come into contact with water, there is the rapid evolution of hydrogen, as the water dissociates to Li—OH+H+. U.S. Pat. No. 5,212,343 teaches the addition of aluminum, lithium (and other elements as well) to water to initiate an explosive reaction. The exothermic reaction of these elements (particularly aluminum and lithium) in water produces large amounts of hydrogen gas, typically 14 cubic centimeters of hydrogen gas is generated per one gram of molten aluminum lithium alloy exposed to water (Ref: U.S. Department of Energy funded research under contract number #DE-AC09-89SR18035). The first claim of U.S. Pat. No. 5,212,343 describes the method to perform this intense interaction for producing a water explosion via the exothermic reaction. This patent describes that with the addition of elements such as lithium a high energy of reaction per unit volume of materials is achieved. As described in U.S. Pat. Nos. 5,212,343 and 5,404,813, the addition of lithium (or some other chemically active element) promotes explosions. These patents teach a process where an explosive reaction is a desirable outcome. These patents reinforce the explosiveness of the addition of lithium to the “bleed-out” or “run-out”, as compared to aluminum alloys without lithium.
The purpose of the modified casting pit design as described in U.S. Pat. No. 4,651,804 is to minimize the potential of an explosion at the bottom of the casting pit when a “bleed-out” or “run-out” occurs during casting of Al—Li alloys. This technique continues to use the coolant water to cool the molds and cool the ingot shell, even after a bleed-out. If the coolant is turned off there is a potential for more serious problems with a melt-through of the molds or additional melt-throughs of the ingot shell causing additional potential for explosions when molten aluminum-lithium and water come into contact. Leaving the water coolant running after a “bleed-out” or “run-out” has occurred has two distinct disadvantages: 1) potential for a molten metal water explosion at various locations near the top of the casting pit or in the ingot crater; 2) potential for a hydrogen explosion because of the generation of H2 as discussed above.
In another method to conducting direct chill casting, patents have been issued related to casting Al—LI alloys using an ingot coolant other than water to provide ingot cooling without the water-lithium reaction from a “bleed-out” or “run-out”. U.S. Pat. No. 4,593,745 describes using a halogenated hydrocarbon or halogenated alcohol. U.S. Pat. Nos. 4,610,295; 4,709,740 and 4,724,887 describe the use of ethylene glycol as the ingot coolant. For this to work, the halogenated hydrocarbon (typically ethylene glycol) must be free of water and water vapor. This is a solution to the explosion hazard, but also introduces a strong fire hazard and is costly to implement and maintain. A fire suppression system is required within the casting pit to contain potential glycol fires. A typical cost to implement a glycol based ingot coolant system including a glycol handling system, a thermal oxidizer to de-hydrate the glycol, and a casting pit fire protection system is on the order of $5 to $8 million dollars (in today's dollars). Casting with 100% glycol as a coolant also brings in another issue. The cooling capability of glycol or other halogenated hydrocarbons is different than that for water, and different casting practices as well as casting tooling are required to utilize this technology. Another disadvantage affiliated with using glycol as a straight coolant is that because glycol has a lower heat conductivity and surface heat transfer coefficient than water, the microstructure of the metal cast with 100% glycol as a coolant tends to have coarser undesirable metallurgical constituents and exhibits higher amount of centerline shrinkage porosity in the cast product. Absence of finer microstructure and simultaneous presence of higher concentration of shrinkage porosity has a deleterious effect on the properties of the end products manufactured from such initial stock.
In yet another case, described in U.S. Pat. No. 4,237,961, the water is removed from the ingot during direct chill casting. In European Patent No. 0-183-563, a device is described for collecting the “break-out” or “run-out” molten metal during direct chill casting of aluminum alloys. Collecting the “break-out” or “run-out” molten metal concentrates this mass of molten metal. This teaching cannot be used for Al—Li casting since it would create an artificial explosion condition where removal of the water would result in a pooling of the water as it is being collected for removal. During a “bleed-out” or “run-out” of the molten metal, the “bleed-out” material would also be concentrated in the pooled water area. As taught in U.S. Pat. No. 5,212,343, this would be a preferred way to create a reactive water/Al—Li explosion.
Accordingly, there remains a significant need for improved apparatus and processes to further minimize the potential for explosions in the direct chill casting of Al—Li alloys and to simultaneously produce a higher quality of the cast product.